Refactor: bundle ut/cpp shared sources into OBJECT libraries

[ChaoWao]

Summary

• tests/ut/cpp/CMakeLists.txt listed the same runtime sources in every
  add_executable(), so on every clean build the 9 hierarchical sources
  (types.cpp, tensormap.cpp, ring.cpp, scope.cpp, orchestrator.cpp,
  worker_manager.cpp, scheduler.cpp, worker.cpp, chip_worker.cpp)
  recompiled 5x and the 5 a2a3 runtime sources (pto_ring_buffer.cpp,
  pto_shared_memory.cpp, pto_scheduler.cpp, pto_tensormap.cpp,
  test_stubs.cpp) recompiled 10x — about 41 redundant Building CXX object
  lines per build.
• Hoist the shared sources into two OBJECT libraries —
  hierarchical_objs and a2a3_rt_objs — that carry the include
  directories and -D_GLIBCXX_USE_CXX11_ABI=0 flag as PUBLIC. Each test
  executable links the OBJECT lib instead of relisting the sources.
• Collapse the 10 add_a2a3_runtime_test(... SOURCES ... EXTRA_SOURCES ${A2A3_RUNTIME_SOURCES})
  call sites to plain (name src) one-liners.

Per-test executables, ctest labels (no_hardware, requires_hardware_a2a3),
and the CI label filters (-L/-LE requires_hardware) are all unchanged,
so both the no-hw ut job and the hw ut-a2a3 / ut-a5 jobs pick up
exactly the same set of tests as before.

Testing

• cmake -B build -S tests/ut/cpp && cmake --build build -j on macOS
  (AppleClang) — clean build, every test target links.
• Spot-checked test_tensormap (9/9 passed) and test_task_allocator
  (22/22 passed) — confirms the OBJECT libs link into runnable binaries.
• Linux CI (ut, ut-a2a3, ut-a5) — relies on the same CMake
  tree; expect the redundant Building CXX object lines to disappear
  from the build log.

[gemini-code-assist]

Code Review

This pull request refactors the CMake configuration for C++ unit tests by introducing OBJECT libraries for shared A2A3 and hierarchical runtime sources. This optimization ensures shared components are compiled once and reused across multiple test executables, improving build performance. The review feedback highlights that the simplified helper functions for adding tests now restrict the number of source files; it is recommended to use ${ARGN} to restore flexibility for tests requiring multiple source files or mocks.